1. Field of the Invention
The present invention relates to the field of drilling, completion, and repair of oil and gas wells and more particularly to wrenches for screwing a drill pipe string together and apart.
2. Description of the Prior Art
The operation of screwing the threaded connections together or apart is the major one in the overall set of operations associated with hoisting a drill pipe string from, or lowering it into, a well, the major one in terms of labor input, time, and complexity of control.
Pipe wrenches extensively employed in the industry comprise as a rule a high-torque device, a low-torque device, and a locking device, each of the devices comprising grippers and an actuator which provides moving the grippers together and apart.
The high-torque device, intended for a final tightening of the thread in screwing a joint together and for an initial loosening of the joint in screwing it apart, comprises in addition a rotation actuator which serves to rotate a pipe with respect to the drill string through an angle constituting a portion of a full revolution.
The low-torque device, intended for a preliminary screwing-in of the pipe and for a final screwing-off of it, comprises a spinning actuator (see, e.g., U.S. Pat. No. 3,545,313, Cl. 81-57, 34, 1970, No. 4,023,449, Cl. 81-57, 1977).
Wrenches of such a construction are dependable and convenient in use, but suffer from an inadequate efficiency in terms of the speed of performing the screwing together/apart operation, which is primarily due to the necessity of employing manual procedures, stemming in particular from different states of the threaded joints. The greatest difficulty in developing constructions of automated pipe wrenches is posed by detecting the end of thread tightening in screwing a joint together and the end of the initial loosening of thread in screwing a joint apart.
In prior attempts to develop automatic wrenches such a detecting has been effected from one of characteristics, such as an axial displacement of the pipe being screwed in or off (see, e.g., USSR Inventor's Certificate No. 629,314, Int. Cl.sup.2. E21B, 19/16, 1978).
Because of a very small axial displacement of the pipe during the initial loosening or tightening of the thread (this amount is less than the thread pitch), the detecting or sensing based on this characteristic cannot be accurate even with an ideal state of the threaded joint.
The lack of accuracy in detecting the initial loosening of thread necessitates stopping the operation of the wrench and repeating the operation manually, which cuts down its efficiency. The lack of accuracy in detecting the thread tightening impairs the dependability of the threaded joint and thereby creates a hazard for the attending personnel and may result in a grave failure of the well equipment.